1. Field of the Invention
This invention relates generally to circuitry for multiplying signal frequency. More particularly this invention relates to a self calibrating frequency multiplier which is operable with an accurate low frequency signal source and a less accurate high frequency clock source to provide economical yet accurate high frequency output.
2. Description of the Prior Art
Frequency multiplication has been disclosed using reference oscillators to provide multiplication of an input signal source. Particularly, the traditional system uses a first reference oscillator to drive a first counter to measure the input signal period. A second reference oscillator is used to drive a second counter, and when the second counter resets it generates an output pulse. The count at which the second counter resets is controlled by the count of the first counter. This type of system was described in U.S. Pat. No. 3,798,584. Later an improved system eliminated the second reference oscillator by using a look-up table to determine the reset count. This modification is described in U.S. Pat. No. 3,970,954. The attractiveness of multipliers of this type is that they track the input signal closely and respond quickly to changes in the input frequency. In these designs, however, relatively accurate reference oscillators have been required and no provision is made to synchronize or calibrate the output relative to the input.